Most soda ash (sodium carbonate) produced in the United States is obtained from naturally occurring subterranean trona ore deposits located in southwestern Wyoming. Trona ore consists mainly of sodium sesquicarbonate, Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O, a hydrated sodium carbonate, sodium bicarbonate double salt.
At the present time, these trona deposits are mechanically mined and the dry-mined trona ore converted to soda ash by either the sesquicarbonate process or the monohydrate process. In the sesquicarbonate process, the trona ore is dissolved in hot aqueous alkali solution and, after separation of the resulting solution from the insolubles, sodium sesquicarbonate is crystallized from solution by cooling. The sesquicarbonate crystals are separated from the mother liquor and then calcined to recover soda ash.
In the monohydrate process, the dry-mined trona ore is first calcined to convert its bicarbonate content to sodium carbonate and is then dissolved in water. After the resulting solution is separated from the insolubles, sodium carbonate monohydrate is precipitated by evaporative crystallization. The monohydrate crystals are separated from the mother liquor and dried to recover soda ash.
Solution mining techniques have been proposed as an alternative to mechanical mining for exploiting trona ore deposits but have not yet been used commercially to recover soda ash. Representative solution mining processes are disclosed in U.S. Pat. Nos. 3,184,287 and 3,953,073, both of which employ an aqueous sodium hydroxide solution as the mining fluid; and in U.S. Pat. No. 2,388,009, which employs a hot carbonate solution as the mining fluid. These prior art solution mining processes, however, involve substantial energy inputs, in manufacturing caustic soda and in supplying the requisite high solution temperature, respectively.
In addition to trona, nahcolite (predominantly NaHCO.sub.3) and wegscheiderite (predominantly Na.sub.2 CO.sub.3.3NaHCO.sub.3) are also sodium bicarbonate-containing ores from which it is possible to recover soda ash, after conversion of the bicarbonate to carbonate. Known deposits of nahcolite and wegscheiderite are located primarily in Utah and Colorado. No commercial operations are presently known to be recovering soda ash from these NaHCO.sub.3 -bearing minerals.
The present invention provides an economical method of recovering alkali values from dry-mined or solution-mined sodium bicarbonate-containing ores through the use of ammonia.
Ammonia is ordinarily associated with soda ash production in the context of the Solvay or ammonia-soda process. In the Solvay process, soda ash is prepared from sodium chloride, ammonia, carbon dioxide and lime by a series of reactions which provide for virtually complete recovery and reuse of the ammonia and unused carbon dioxide. Environmental concerns and high costs have led to the demise of the Solvay process for manufacturing soda ash in the United States, where abundant deposits of naturally occurring trona have provided an alternative source for soda ash.
Ammonia has not previously been used in processes involving the recovery of soda ash from naturally occurring trona ore deposits, either in the monohydrate or sesquicarbonate processes for preparing soda ash from dry-mined trona ore or in prior art solution mining techniques disclosed as being applicable to trona.
The present invention employs ammonia in a novel fashion to recover alkali values, preferably as soda ash, from naturally occurring sodium bicarbonate-containing ore deposits.